The present invention relates to a magnetic recording medium and a process for producing the magnetic recording medium, and more particularly, to a magnetic recording medium comprising a spinel-type iron oxide thin film comprising maghemite as a main component that is constituted by grains having a small average grain size and an excellent grain size distribution, and exhibits a high coercive force and an excellent squareness, and a process for producing the magnetic recording medium in an industrially and economically useful manner.
In recent years, in magnetic recording apparatuses, it has been more increasingly demanded to provide magnetic recording media having a high-density recording property and a high reliability in order to deal with a large capacity data such as various back-up data, images and audio data. Especially, in hard disk drives, there has been a remarkable tendency of a miniaturization and a high reliability in accordance with information devices being miniaturized and systems used therefor being required to have a high reliability.
In order to satisfy such properties, the magnetic recording media have been strongly required to have not only a high coercive force, but also exhibit a less noise, an excellent durability and a reduced distance (magnetic spacing) between a magnetic recording layer and a magnetic head.
As magnetic recording media having a high coercive force, there are widely known magnetic recording media comprising a substrate and a magnetic thin film formed on the substrate, in case of the hard disk drives.
The magnetic thin films practically used in magnetic recording media are generally classified into magnetic alloy thin films such as Coxe2x80x94Cr-based alloy thin films and magnetic oxide thin films such as spinel-type iron oxide thin film comprising maghemite as a main component.
The magnetic alloy thin films such as Coxe2x80x94Cr-based alloy thin films, have a coercive force as high as not less than 159 kA/m (2,000 Oe). However, these alloy materials themselves tend to be readily oxidized and, therefore, deteriorated in stability as well as magnetic properties with the passage of time.
In order to prevent the deterioration of magnetic properties due to oxidation and enhance the durability, the surface of the magnetic alloy thin film is coated with a protective film composed of carbon, SiO2 or the like having a thickness of usually about 5 to 10 nm, resulting in undesired increase in magnetic spacing corresponding to the thickness of the protective layer.
On the other hand, the magnetic oxide thin films such as spinel-type iron oxide thin film comprising maghemite as a main component are excellent in oxidation resistance or corrosion resistance due to inherent properties of oxides. For this reason, the magnetic oxide thin films can show an excellent stability independent of the passage of time, and a less change in magnetic properties with the passage of time. Further, since the oxides exhibit a higher hardness as compared to metals, the magnetic oxide thin films can also show an excellent durability. As a result, in the magnetic oxide thin film, the thickness of a protective layer which is essential to the magnetic alloy thin film can be reduced or the protect layer can be omitted. Therefore, the magnetic oxide thin films can exhibit a small magnetic spacing as compared to the magnetic alloy thin films, so that the magnetic oxide thin films are optimum ones for the production of high-density magnetic recording media.
It has been attempted to enhance a coercive force of the spinel-type iron oxide thin film comprising maghemite as a main component by incorporating cobalt thereinto. However, with the increase in cobalt content, the spinel-type iron oxide thin film comprising maghemite as a main component tend to be deteriorated in stability with the passage of time due to adverse influences of heat or the like.
With the increasing demand for providing magnetic recording media capable of high-density recording, it has been required to enhance a coercive force of the magnetic recording media. In addition, in order to reduce the noise due to the media themselves, the magnetic recording media have been required to have a thin magnetic layer and a smooth surface. Further, in order to obtain magnetic recording media having a high recording resolution such as an excellent overwrite characteristics or the like, the magnetic recording media have been required to exhibit a high squareness. The squareness mentioned in the present invention means a coercive squareness ratio S* (referred in Magnetic Charecterisation of Thin Film Recording Media, published in IEEE TRANSACTIONS ON MAGNETICS, Vol. 29, No. 1, Jan. 1, 1993, pages 286-289) in the case of a longitudinal recording medium, and a squareness ratio S (Mr/Ms) in the case of perpendicular recording medium.
The noises caused from magnetic recording media are mainly classified into transition noises and modulation noises. The transition noises depend upon the size of magnetic grains constituting the magnetic thin film. Therefore, it has been required to reduce an average grain size of grains constituting the magnetic thin film.
Also, in the case where the magnetic recording media have a less smooth surface, modulation noises are caused therefrom. Therefore, in order to reduce the modulation noises, it is necessary to improve a surface smoothness of the magnetic thin film.
On the other hand, in order to reduce the magnetic spacing of magnetic recording media, it is required to minimize flying height of a magnetic head therefrom, and always allow the magnetic head to be flying stably. In conventional hard disk drives, magnetic recording media used therefor have been required to have a certain surface roughness in order to prevent the magnetic head from being absorbed thereon owing to a meniscus force therebetween upon stopping the magnetic head. However, as a result of current improvement in these hard disc systems, magnetic recording media have been no longer required to have such a surface roughness for preventing the magnetic head from being absorbed thereon. Therefore, it has been demanded that a magnetic thin film used in these magnetic recording media exhibits a more excellent surface smoothness.
At present, in the magnetic recording media using a magnetic oxide thin film, since the magnetic thin film has a thickness as small as 50 nm or less, the surface property of the magnetic thin film considerably depends upon that of a substrate. As a result, it has been required not only to use a substrate having an excellent surface smoothness, but also to develop techniques for further smoothening the surface of the magnetic thin film.
As conventional methods for producing spinel-type iron oxide thin film comprising maghemite as a main component, there are known, for example, (1) a method of forming a hematite thin film on a substrate, reducing the hematite thin film at a temperature of 230 to 320xc2x0 C. to transform the hematite thin film into a spinel-type iron oxide thin film comprising magnetite as a main component, and then oxidizing the spinel-type iron oxide thin film comprising magnetite as a main component at a temperature of 290 to 330xc2x0 C.; (2) a method of forming a spinel-type iron oxide thin film comprising magnetite as a main component on a substrate, and then oxidizing the spinel-type iron oxide thin film comprising magnetite as a main component at a temperature of 280 to 350xc2x0 C.; or the like.
Furthermore, as the conventional methods for producing a spinel-type iron oxide thin film comprising maghemite as a main component, there may be exemplified the following methods in addition to the above-mentioned methods (1) and (2):
(3) a method of forming a spinel-type iron oxide thin film on an underlayer composed of Cr, V or the like (Japanese Patent Publication (KOKOKU) No. 55-21451(1980));
(4) a method of conducting a sputtering treatment using a target containing Fe3O4 as a main component in an inert gas atmosphere containing 1.5 to 5% by volume of oxygen to directly form a film of xcex3-Fe2O3 (Japanese Patent Publication (KOKOKU) No. 62-49724(1987));
(5) a method of decomposing a vapor of a metal chelate, a metal carbonyl or ferrocenes containing Fe, and a metal chelate, a metal carbonyl or ferrocenes containing M, wherein M represents at least one metal selected from the group consisting of Co, Cu, Rh, Ru, Os, Ti, V and Nb; together with an oxygen gas in a high-density pressure-reduced plasma by applying a magnetic field thereto, thereby directly forming a spinel-type iron oxide thin film comprising maghemite as a main component, indicated by a chemical formula: (Fe1xe2x88x92xMx)2O3; and x is a number of 0.01 to 0.1, on a substrate (Japanese Patent Application Laid-Open (KOKAI) No. 3-78114 (1991));
(6) a method of treating a spinel-type iron oxide thin film comprising magnetite as a main component at a temperature of less than 240xc2x0 C. using an electron cyclotron resonance (ECR) plasma to transform the spinel-type iron oxide thin film comprising magnetite as a main component into a spinel-type iron oxide thin film comprising maghemite as a main component (Japanese Patent Application Laid-Open (KOKAI) No. 11-328652(1999));
(7) a method of subjecting a spinel-type iron oxide thin film comprising magnetite as a main component to a plasma treatment in an oxygen atmosphere, and then conducting a heat treatment in air to transform the spinel-type iron oxide thin film comprising magnetite as a main component into a spinel-type iron oxide thin film comprising maghemite as a main component (EP 1089262A1); or the like.
Also, spinel-type iron oxide thin films comprising maghemite as a main component having a small average grain size are described in Japanese Patent Application Laid-Open (KOKAI) Nos. 3-78114(1991) and 4-117624(1992), Japanese Patent Publication (KOKOKU) No. 6-61130(1994), Japanese Patents Nos. 2840966, 2829524 and 2983053, etc.
As described in xe2x80x9cCeramicsxe2x80x9d, vol. 24, No. 1 (1989), pp. 22-23, a relationship between the film deposition rate and oxygen partial pressure, when an iron oxide compound thin film by a sputtering method, is known, and it indicates that hematite (xcex1-Fe2O3) is deposited with low deposition rate in higher the oxygen partial pressure.
At present, it has been strongly required to provide magnetic recording media comprising a spinel-type iron oxide thin film comprising maghemite as a main component, which is constituted by grains having a small average grain size and an excellent grain size distribution, and exhibits a high coercive force. However, the conventional magnetic recording media fail to satisfy these properties.
Namely, in the above methods (1) and (2), the spinel-type iron oxide thin film comprising magnetite as a main component is taken out into atmospheric air, and further subjected to oxidation treatment at a temperature of 290 to 450xc2x0 C., thereby obtaining the spinel-type iron oxide thin film comprising maghemite as a main component. However, in these methods, since it is necessary to conduct the heat treatment at a temperature as high as not less than 290xc2x0 C., there arises such a problem that the obtained thin film is deteriorated in magnetic properties such as a coercive force, squareness or the like, due to migration from the substrate and the underlayer, and the grains contained in the thin film tend to be increased in grain size. Further, since a material of the substrate must be selected from those having an excellent heat resistance, the substrate usable for the thin film is limited to specific ones. In addition, since the thin film is taken out into atmospheric air, there also arise additional problems such as the contamination on the thin film by dust particles.
In the method (3), the obtained spinel-type iron oxide thin film comprising maghemite as a main component fails to show a sufficiently high coercive force required for magnetic recording media despite excellent squareness such as coercive squareness ratio S* thereof.
In the method (4), since the spinel-type iron oxide thin film comprising maghemite as a main component can be produced at a temperature of not more than 100xc2x0 C., a substrate made of a plastic material such as PET is applicable thereto. However, the coercive force of the obtained magnetic recording medium is as low as not more than 81.2 kA/m (1,020 Oe), thereby failing to provide a magnetic recording medium having a sufficiently high coercive force.
In the method (5) for producing the cobalt-containing spinel-type iron oxide thin film comprising maghemite as a main component, since such a spinel-type iron oxide thin film comprising maghemite as a main component can be produced at a substrate temperature of about 50xc2x0 C., a substrate made of a plastic material having a less heat resistance, such as polyester, polystyrene terephthalate and polyamide, can be used therefor. However, the resultant magnetic recording medium exhibits a coercive force of about 135 kA/m (1,700 Oe) at most.
In the method (6), the spinel-type iron oxide thin film comprising maghemite as a main component can be produced at a treating temperature of less than 240xc2x0 C. only by a vacuum process and, therefore, can be prevented from undergoing adverse influences such as contamination and migration, resulting in production of magnetic recording medium having an excellent surface smoothness. However, the obtained magnetic recording medium fails to show a sufficiently high squareness.
In the method (7), it is possible to produce a magnetic recording medium having an excellent surface smoothness and a high coercive force. However, since the magnetic recording medium is heat-treated in atmospheric air, it is difficult to inhibit the migration from the underlayer and the substrate. In addition, as shown in the below-mentioned Comparative Examples, the obtained magnetic recording medium fails to show an excellent squareness such as coercive squareness ratio S*.
Further, in the methods described in Japanese Patent Application Laid-Open (KOKAI) Nos. 3-78114(1991) and 4-117624(1992) and Japanese Patent Publication (KOKOKU) No. 6-61130(1994), it is possible to produce a spinel-type iron oxide thin film comprising maghemite as a main component having a thickness as small as not more than 50 nm. However, the obtained spinel-type iron oxide thin film comprising maghemite as a main component fails to exhibit a sufficiently high coercive force. Furthermore, in the methods described in Japanese Patent Nos. 2840966, 2829524 and 2983053, it is possible to produce a spinel-type iron oxide thin film comprising maghemite as a main component, which is constituted by grains having a small average grain size, and has a thickness as small as not more than 50 nm. However, the obtained magnetic recording medium fails to show a sufficiently high coercive force.
As a result of the present inventors"" earnest studies for solving the above problems, it has been found that by forming an underlayer on a substrate, after conducting a sputtering treatment in an oxygen-rich atmosphere, conducting a reactive sputtering to form a spinel-type iron oxide thin film comprising magnetite as a main component on the underlayer, and then oxidizing the spinel-type iron oxide thin film comprising magnetite as a main component to transform the spinel-type iron oxide thin film into a spinel-type iron oxide thin film comprising maghemite as a main component, the obtained spinel-type iron oxide thin film comprising maghemite as a main component is constituted by grains having a small average grain size and an excellent grain size distribution, and exhibits a high coercive force and an excellent squareness with a thickness (t) as small as 5 to 50 nm. The present invention has been attained based on the above finding.
An object of the present invention is to provide a magnetic recording medium comprising a spinel-type iron oxide thin film comprising maghemite as a main component, which is constituted by grains having a small average grain size and an excellent grain size distribution, and exhibits a high coercive force and a high squareness.
Another object of the present invention is to provide a process for producing a magnetic recording medium comprising a spinel-type iron oxide thin film comprising maghemite as a main component, which is constituted by grains having a small average grain size and an excellent grain size distribution, and exhibits a high coercive force and a high squareness.
To accomplish the aims, in the first aspect of the present invention, there is provided a magnetic recording medium, comprising:
a substrate;
an underlayer formed on the substrate; and
a spinel-type iron oxide thin film comprising maghemite as a main component formed on the underlayer,
said spinel-type iron oxide thin film having a thickness (t) of 5 to 50 nm, being constituted by grains having an average grain size (D) of 5 to 30 nm, a standard deviation of grain sizes of not more than 4 nm and a ratio (D/t) of the average grain size (D) to the thickness (t) of less than 1.0, and exhibiting a coercive force of not less than 159 kA/m (2,000 Oe) and a coercive squareness ratio S* of not less than 0.5:1 in a longitudinal recording medium or a squareness ratio not less than 0.75:1 in a perpendicular recording medium.
In a second aspect of the present invention, there is provided a magnetic recording medium, having a center-line mean (Ra) of 0.1 to 1.0 nm and a maximum height (Rmax) of 1 to 12 nm, comprising:
a substrate;
an underlayer formed on the substrate; and
a spinel-type iron oxide thin film comprising maghemite as a main component formed on the underlayer,
said spinel-type iron oxide thin film having a thickness (t) of 5 to 50 nm, being constituted by grains having an average grain size (D) of 5 to 30 nm, a standard deviation of grain sizes of not more than 4 nm and a ratio (D/t) of the average grain size (D) to the thickness (t) of less than 1.0, and exhibiting a coercive force of not less than 159 kA/m (2,000 Oe) and a coercive squareness ratio S* of not less than 0.5:1 in a longitudinal recording medium or a squareness ratio not less than 0.75:1 in a perpendicular recording medium.
In a third aspect of the present invention, there is provided a process for producing a magnetic recording medium, comprising:
forming an underlayer on a substrate;
subjecting the underlayer to sputtering treatment in an oxygen-rich atmosphere using an Fe target, an Fe alloy target or a target comprising iron oxide as a main component;
conducting a reactive sputtering using an Fe target, an Fe alloy target or a sputtering using a target comprising iron oxide as a main component to form a spinel-type iron oxide thin film comprising magnetite as a main component on the underlayer; and
oxidizing the spinel-type iron oxide thin film comprising magnetite as a main component to transform into a spinel-type iron oxide thin film comprising maghemite as a main component.
In a fourth aspect of the present invention, there is provided a process for producing a magnetic recording medium, comprising:
forming an underlayer comprising a Cr metal thin film or a Cr alloy thin film on a substrate;
subjecting the underlayer to sputtering treatment in an oxygen-rich atmosphere using an Fe target, an Fe alloy target or a target comprising iron oxide as a main component; and
conducting a reactive sputtering using an Fe target, an Fe alloy target or a sputtering using a target comprising iron oxide as a main component to form a spinel-type iron oxide thin film comprising maghemite as a main component on the underlayer.